docs: def validate(key, val):
      """ Validate a configuration value.
      """
      if val and val.startswith("~/"):
          return os.path.expanduser(val)
      if key == "output_header_frequency":
          return int(val, 10)
      if key.endswith("_ecma48"):
          return eval("'%s'" % val.replace("'", r"\'"))  # pylint: disable=eval-used
  
      return val